Localization

1- There's an issue with webdav.js, cf. lines 29-31. I wasn't able to fix the startup error provoked by the localization (tried to mimick <a href="https://github.com/zotero/zotero/commit/3fb81ab1b93d9f5650690820b92e5a7c5726b47e">this change</a>, without success).

2- I've created a merge.dtd file since there was an existing reference to it in merge.xul and merge.xml. I'm not sure it's worth a specific file, though.

3- I've noticed that http://www.zotero.org/support/kb/encrypted_filenames does not exist (the users are advised to visit this page in an error message).

4- I'm not exactly sure of the difference between sync.js and merge.js regarding to conflicts messages... Then, I put all the strings under 'sync.conflict.xxx'


I've been careful and tried to test the localization as much as possible but, in some cases, I wasn't able to get the relevant error messages (eg. file.js)
Hopefully, this won't break anything!